In general, electrophotographing toner consisting of a binder resin, a coloring agent, a charge-controlling agent, etc. is used in the electrophotographic process. When such electrophotographing toner is manufactured, materials such as a binder resin, a coloring agent, a charge-controlling agent, a mold releasing agent and a lubricant are first mixed in a mixer, and the resulting mixture is melt-kneaded by a two-shaft extrusion-type melt-kneader, and then cooled off so as to preliminarily produce a plate-shaped toner in a solid state. In a conventional process, this toner is further ground into a predetermined particle diameter by a grinding method using a collision plate so as to form electrophotographing toner.
Resins such as polyester resin and styrene-acryl resin are generally used as the binder resin. Nigrosine dye is generally used as the charge-controlling agent. Carbon black, etc. is commonly used as the coloring agent.
In a conventional electrophotographing method using the dry-type developing system, the heat-roll fixing system is generally adopted, in which after an electrostatic latent image has been developed by the toner, it is fixed by being heated and pressed by a heating roller. However, the disadvantage with this method is that some of the toner adheres to the heating roller from the transferring sheet and further contaminates a new transfer sheet that has been transported thereto, resulting in a so-called offset phenomenon.
In order to prevent the offset phenomenon, wax (of the olefin family) is conventionally added to the electrophotographing toner so as to improve its mold-releasing and lubricating properties. Further, wax is often added to the electrophotographing toner for the purpose of easily cleaning the electrophotographing toner from the toner-bearing body.
For example, in order to improve the cleaning performance of the toner, Japanese Laid-Open Patent Publication No. 156958/1980 (Tokukaishou 55-156958) discloses toner to which polyolefin wax having a viscosity within a predetermined range is added.
Moreover, Japanese Examined Patent Publication No. 12447/1996 (Tokukouhei 8-12447) discloses that toner to which polyethylene wax is added has a superior cleaning performance for an organic photoconductor.
However, in the case when polyethylene wax (of the olefin family) is merely added to toner as a mold-releasing agent and a lubricant, the compatibility between the binder resin and the polyethylene wax badly deteriorates, with the result that the polyethylene wax is hardly dispersed into the binder resin, resulting in separated polyethylene wax particles outside the toner particles.
When separated polyethylene wax molecules are produced outside the toner particles, the following problems arise: the charging property of the toner becomes unstable, reducing the image density; the separated polyethylene wax particles badly reduce the fluidity of the toner; and the service life of the toner and the toner-bearing body is shortened due to wax contamination in which the separated polyethylene wax particles contaminate the surfaces of the carrier and the toner-bearing body such as the developing cylinder.
In order to avoid the above-mentioned problems, Japanese Examined Patent Publication No. 12447/1996 (Tokukouhei 8-12447) discloses toner which is made of at least a binder resin and a coloring agent and contains polyethylene wax at a ratio of 0.5 to 10% by weight. In this toner, the number of polyethylene wax particles that have a size of not less than 1 .mu.m and that are separated outside toner particles is set at not more than 10 per 100 toner particles.
Further, the above-mentioned patent publication also discloses a manufacturing method of toner in which, under a condition that the melt viscosity of the binder resin is not less than 100 Pa.multidot.s, the resin, the coloring agent and polyethylene wax are melt-kneaded. When these materials are melt-kneaded under the above-mentioned condition, the binder resin exerts a high viscosity shearing force on the polyethylene wax during the melt-kneading process so that the polyethylene wax is allowed to form fine particles and are dispersed inside the binder resin.
However, the above-mentioned arrangement merely limits the number of polyethylene wax particles that have large diameters and that are separated outside toner particles, and fails to disclose anything about polyethylene wax particles inside the toner particles (including the surface thereof).
If wax particles having large particle diameters exceeding 6 .mu.m are contained in the toner particles, the wax particles, having large particle diameters existing in the toner particles, tend to expose themselves to the toner surface under high-temperature and high-moisture conditions, causing contamination on the surface of the toner-bearing body in the same manner as separated wax particles having large diameters.
Moreover, if the shape of wax particles is represented by a ratio of major axis/minor axis indicating a shape such as a needle, the wax particles tend to stick out from the toner surface, thereby causing contamination on the surface of the toner-bearing body in the same manner as separated wax particles having large diameters.
Furthermore, in the toner as described in the above-mentioned prior-art publication, when the melt-kneading process is carried out under a condition in which the wax in the olefin family comes to have a viscosity allowing easy dispersion, the coloring agent tends to re-aggregate to form secondary particles, thereby resulting in degradation in the dispersing property of the coloring agent and the subsequent instability or degradation in the charging property. For this reason, in the above-mentioned prior art, the toner, which has a reduced charging property and the subsequent reduced fluidity, is further subjected to reduction in the fluidity due to being left at high temperatures, resulting in high possibilities of toner scattering, fog, etc. during the printing process in a copying machine.